1. Field of the Invention
The present invention relates to a digital satellite communication apparatus which has an audio echo canceler.
2. Description of the Related Art
As shown in FIG. 1, the conventional satellite communication apparatus has an audio echo canceler 301, an audio encoder 308 for encoding an audio signal E1, which is outputted from the audio echo canceler 301 into an encoded audio signal in a format that has been determined for an individual satellite circuit, a modulator 310 for modulating the encoded audio signal from the audio encoder 308 according to a modulation process determined for an individual satellite circuit and converting the modulated audio signal into an audio signal having a radio frequency, an antenna 313 for transmitting the audio signal from the modulator 310, and receiving an audio signal transmitted from an external source, a demodulator 311 for demodulating an audio signal received by the antenna 313 and generating and outputting synchronizing frame data R2 and a synchronizing frame signal FP based on the demodulated audio signal, an audio decoder 309 for decoding the synchronizing frame data R2 and the synchronizing frame signal FP outputted from the demodulator 311 into a digital audio signal X and outputting the digital audio signal X to the echo canceler 301, and a branching filter 312 for outputting the audio signal from the modulator 310 to the antenna 313 and outputting the audio signal from the antenna 313 to the demodulator 311.
The echo canceler 301 comprises a tap coefficient memory 305 for generating and outputting N tap coefficients HR, a received audio signal detector 307 for being supplied with the digital audio signal X from the audio decoder 309, determining whether there is a received audio signal or not, and outputting a detected result, a double-talk detector 306 for being supplied with an audio signal Y generated in a transmission circuit 2 and the digital audio signal X, and, if the audio signal Y and the digital audio signal X are simultaneously present, outputting a signal indicative of such a simultaneous presence, an adaptive filter 302 for generating and outputting a quasi-echo signal HX based on the tap coefficients HR outputted from the tap coefficient memory 305, and outputting a signal HW to estimate tap coefficients for the tap coefficient memory 305 based on the detected result from the received audio signal detector 307 and a signal outputted from the double-talk detector 306, and a subtractor 303 for being supplied with the audio signal Y generated in the circuit 2 and the quasi-echo signal HX from the adaptive filter 302, and subtracting the quasi-echo signal HX from the audio signal Y to generate the audio signal E1 to be transmitted.
As shown in FIG. 2 of the accompanying drawings, the demodulator 311 comprises a frequency converter 403 for converting the frequency of a signal received by the antenna 313 and inputted through the branching filter 312 (see FIG. 1), a demodulating circuit 402 for demodulating the frequency-converted signal from the frequency converter 403 into a digital baseband signal R1, and a frame synchronizing circuit 401 for detecting a pattern indicative of the first timing of effective data from the digital baseband signal R1 outputted from the demodulating circuit 402, and generating and outputting synchronizing frame data R2 and a synchronizing frame signal FP from the detected pattern.
Operation of the conventional satellite communication apparatus will be described below.
In a transmission mode, the conventional satellite communication apparatus operates as follows:
When an audio signal E1 to be transmitted is outputted from the echo canceler 301, the audio signal E1 is supplied to the audio encoder 308. The audio encoder 308 encodes the audio signal E1 into an encoded audio signal in a format that has been determined for the individual satellite circuit, and outputs the encoded audio signal to the modulator 310.
The modulator 310 modulates the encoded audio signal according to a modulation process determined for the individual satellite circuit and converts the modulated audio signal into an audio signal having a radio frequency.
The audio signal outputted from the modulator 310 is supplied through the branching filter 312 to the antenna 313, from which the audio signal is transmitted toward a satellite (not shown).
A present satellite terminal station, particularly a satellite terminal station that is often operated by a battery, employs a voice activation process (or VOX process) which measures the electric power of the inputted audio signal E1 with the audio encoder 308, and determines that there is an audio signal to be transmitted and transmits the audio signal to another station only when the measured electric power is in excess of a certain threshold value. The illustrated conventional satellite communication apparatus operates on such principles.
In a reception mode, the conventional satellite communication apparatus operates as follows:
When an audio signal from another station is received by the antenna 313, the received audio signal is supplied through the branching filter 312 to the demodulator 311.
In the demodulator 311, the frequency converter 403 converts the frequency of the received audio signal, and the demodulator 402 demodulates the frequency-converted audio signal into a digital baseband signal R1. Then, the frame synchronizing circuit 401 detects a pattern (hereinafter referred to as UW) inherent in the satellite circuit and indicative of the starting of the data from the digital baseband signal R1, and detects the start of the data of each frame from the pattern, thus generating synchronizing frame data R2 and a synchronizing frame signal FP (see FIG. 3 of the accompanying drawings). The synchronizing frame data R2 and the synchronizing frame signal FP are then outputted to the audio decoder 309.
The audio decoder 309 decodes the synchronizing frame data R2 and the synchronizing frame signal FP from the demodulator 311 into a digital audio signal X, which is outputted to the echo canceler 301.
According to the voice activation process, at times other than when UW is not detected due to a reception failure, the frame synchronizing circuit 401 of the demodulator 311 is not synchronized when the party at the other station is not talking, because no signal is being received from the satellite. In this case, the synchronizing frame signal FP is fixed to "0" and the synchronizing frame data R2 (actually not synchronized) is undefined in the frame synchronizing circuit 401.
Operation of the echo canceler 301 will be described below.
When the digital audio signal X is supplied from the audio decoder 309, the digital audio signal X is outputted to the circuit 2 and is also supplied to the adaptive filter 302, the double-talk detector 306, and the received audio signal detector 307.
The adaptive filter 302 calculates a quasi-echo signal HX based on the supplied digital audio signal X and the tap coefficients HR outputted from the tap coefficient memory 305 according to the following equation (1): ##EQU1##
where j represents the degree on a time base and i the number of the tap coefficient.
The double-talk detector 306 determines whether only a transmitted audio signal is present or both transmitted and received signals are present, based on a certain threshold value, from the electric power ratio of the audio signal Y from the circuit 2 and the digital audio signal X. If both transmitted and received signals are present, then the double-talk detector 306 outputs a signal indicative of the stopping of the estimation of tap coefficients to the adaptive filter 302.
The received audio signal detector 307 measures the electric power of the digital audio signal X at all times. If the electric power of the digital audio signal X is smaller than a certain threshold value, then the received audio signal detector 307 determines that there is no received audio signal, and outputs a signal indicative of the stopping of the estimation of tap coefficients to the adaptive filter 302.
When the quasi-echo signal X is generated by the adaptive filter 302, the quasi-echo signal HX is supplied to the subtractor 303, which subtracts the quasi-echo signal HX from the audio signal Y from the circuit 2, and outputs the difference as an audio signal E1 to be transmitted to the circuit 1.
Simultaneously, the audio signal E1 is also supplied to the adaptive filter 302. After having generated the quasi-echo signal HX, the adaptive filter 302 estimates tap coefficients according to the following equation (2): ##EQU2##
where .mu. is a parameter for determining the rate of convergence and stability of an echo suppressing quantity.
For generating a quasi-echo signal HX, the adaptive filter 302 reads the N tap coefficients HR from the tap coefficient memory 305. Then, the adaptive filter 302 generates a quasi-echo signal HX using the N tap coefficients HR. For subsequently estimating tap coefficients HR, the adaptive filter 302 newly estimates tap coefficients HR according to the equation (2). The adaptive filter 302 sends the estimated tap coefficients HR as the signal HW to the tap coefficient memory 305 where they are stored.
Each time a digital audio signal X is applied to the adaptive filter 302, the adaptive filter 302 estimates and updates tap coefficients. The characteristics of the adaptive filter 302 are thus made closer to the characteristics of an actual echo path, so that the audio signal E1 to be transmitted is minimized to prevent an echo signal Y, due to the digital audio signal X, from being transmitted.
The conventional satellite communication apparatus may be incorporated in a circuit arrangement which, as shown in FIG. 4 of the accompanying drawings, cancels an echo, which is produced when a received audio signal regenerated by a loudspeaker 602 is picked up by a hands-free microphone 601, with a quasi-echo signal Y that is generated by an adaptive filter 604 (see Japanese laid-open patent publication No. 1994-13940).
In the satellite communication apparatus shown in FIG. 4, a received audio signal detector 608 determines whether there is a received audio signal or not, as follows:
If it is assumed that a speaker signal (transmitted or received signal) at a sampling period i is represented by a(i), then an autocorrelation function S(j) when the speaker signal a(i) is shifted j times the sampling period is given by: EQU S(j)=.SIGMA.a(i)xa(i+j)/.SIGMA.a(i)xa(i) (3)
Two autocorrelation functions S(j) are determined for each i to range from 1 to N (a value having a period longer than the possible maximum pitch of the speaker signal a(i)).
In a transmitted audio signal detector 607 and a received audio signal detector 608, the parameter j is varied in a given range having a central value corresponding to the average pitch of the speaker signal, for thereby determining a plurality of autocorrelation functions S(j). Then, a maximum value Smax(j), for example, of the autocorrelation functions S(j) is determined. If the maximum value Smax(j) is large, then it is determined that there is a speaker signal (transmitted signal or received signal). If the maximum value Smax(j) is small, then it is determined that there is no speaker signal.
The above conventional satellite communication apparatus suffers the following problems:
(1) If the threshold value for determining whether there is a received signal present or not is not optimally established in the received audio signal detector, and it is determined that a received signal is present and tap coefficients are estimated even though no received signal is present, then the characteristics of the adaptive filter may possibly be degraded. When ambient conditions and background noise are considerably different for communicating stations, e.g., those of portable telephone or mobile satellite communications, depending on how these stations are used, it is considerably difficult to set the threshold value to an optimum level, and it takes a considerable period of time to search for the threshold value.
(2) In the event of a reception failure in satellite circuits, undefined data is outputted from the demodulator to the audio decoder, which tends to operate in error and generate noise. When the generated noise is applied to the subtractor, it degrades the characteristics of the adaptive filter.